An installation for transmission of high-voltage direct current between two alternating-voltage networks comprises two converter stations, each one connected on its ac side to a separate one of the alternating-voltage networks, and a common dc connection. The dc connection may be in the form of an overhead line and/or a cable and also in certain parts consist of ground or water instead of a metallic conductor. Each one of the converter stations comprises a converter, usually at least one converter transformer for connection of the converter to the alternating-voltage network, as well as shunt filters for generation of reactive power and filtering of harmonics. The converters are normally line-commutated, current-source converters, by which is to be understood that the current commutation between the valves of the converters takes place by means of voltages occurring in the alternating-voltage network, and that the dc connection, viewed from the converters, occurs as a stiff current source.
During normal operation, one of the converters, hereinafter referred to as the rectifier, operates in rectifier operation, and the other, hereinafter referred to as the inverter, operates in inverter operation. Control equipment for the respective converter generates a control signal corresponding to a control angle .alpha. at which firing pulses are applied to the valves of the converters. For the purpose of minimizing the consumption of reactive power by the converters, and reducing the stresses on components included in the converter stations, it is advantageous to control the rectifier with the smallest possible control angle .alpha. and to control the inverter with a control angle which results in the smallest possible extinction angle .gamma. (margin of commutation) without jeopardizing the controlled operation.
The control system of the installation is, therefore, usually designed such that the inverter is controlled to a suitable maximum direct voltage for the operating conditions of the installation, taking into consideration safety margins with respect to commutating errors, voltage variations on the ac network, and other deviations from nominal operation which may occur whereas the rectifier is controlled in current control. The current reference value of the current control is formed in dependence on a current order, which is turn is formed in dependence on a power order and the prevailing direct voltage in such a way that the direct current and hence the transferred active power remain at a desired value. The control angle of the rectifier during stationary operation is chosen as small as possible while taking into consideration that the current control requires a certain control margin with respect to the smallest permissible control angle which is required for maintaining reliable operation.
Usually, the control equipment for a rectifier and an inverter is designed in similar manner, whereby in the rectifier a current controller is activated and in the inverter control equipment for a control with the aim of maintaining the extinction angle at, but not lower than, a preselected lowest value is activated. This is achieved by applying to the current controllers of both the rectifier and the inverter current reference values formed in dependence on the current order for the rectifier and to the current controller of the inverter, in addition, a current margin with a value different from zero and with such a sign that the current controller strives to reduce the direct current controlled by the rectifier. The current margin of the rectifier is given a value equal to zero. The current orders and the current margins for the rectifier and the inverter are coordinated via a telecommunication link. Voltage reductions in the alternating-voltage network of the rectifier may lead to the rectifier not being able to maintain the ordered current. When the current has dropped by an amount corresponding to the current margin of the inverter, the inverter takes over the current control and controls the current on a value equal to the current order reduced by the current margin. The transferred active power in the dc connection thus drops in relation to the amounts of the voltage reduction and of the current margin. The transition process is usually also associated with a transient reduction of the current by a value greater than the current margin.
For a general description of the technique for transmission of high-voltage direct current, reference is made to Erich Uhlmann: Power Transmission by Direct Current, Springer Verlag, Berlin Heidelberg New York 1975, in particular pages 125-140.
To counteract the reduction of transferred power under the above-mentioned circumstances, it has been proposed to use a controller which, when it has been determined that the inverter has taken over the current control, via a feedback control circuit increases the current reference value of the inverter by a value corresponding to the current margin. However, this entails a complication of the control equipment and an often difficult tuning of the dynamic behaviour of the controller.
Thus, it is a desire to reduce the current margin in order to reduce transients and loss of transferred power. The current margin has usually been given values of typically 0.1 per unit, among other things due to the need of coordination of the current order between the rectifier and the inverter, which has often been performed through ordinary telephone communication. With faster and automatically operating telecommunication links, the current margin may be reduced to typically the order of magnitude of 0.02 per unit. This means that unavoidable transitions Of the current control to the inverter are performed with a smaller reduction of the current and with less transients during the transition process.
In series-compensated converter stations, by which are meant converter stations the converter bridges of which are connected to the respective alternating-voltage network via series capacitors, possibly via an intermediate transformer, the risk of the current control being transferred to the inverter increases. The reason for this is that the rectifier in series-compensated converter stations can normally operate with nominal control angles, related to the phase position for the voltages of the alternating-voltage network, which are smaller than those of the non-series-compensated ones, typically with control angles of about 5.degree. instead of about 15.degree.. When the voltage of the rectifier as a function of the control angle .alpha. is substantially proportional to cos.alpha., the direct voltage at nominal control angle for a series-compensated converter station will have a flatter dependence on the control angle, which entails a reduced control margin.
In case of large voltage reductions in the alternating-voltage network of the rectifier, typically greater than 0.1 per unit, for example caused by errors in the alternating-voltage network or, if this is weak, when connecting a large load, certain problems may arise in the coordination of the current reference values of the rectifier and the inverter. The control equipment of the converter normally comprises a limitation of the current order in dependence on the direct voltage at the respective converter, designed such that the current order with decreasing direct voltage is limited to a value decreasing with the voltage and is limited, at a lower voltage level, to a constant value. Current controllers in the control equipment of the converters are supplied with the current order, thus limited, as current reference values. However, it is the current order prior to this limitation that is coordinated, via the telecommunication link, between the rectifier and the inverter. It is probable that, in connection with the above-mentioned large voltage reductions, the voltage-dependent limitations of the current order become effective. Since, however, the direct voltage of the rectifier, particularly in case of errors, may significantly deviate from the direct voltage of the inverter, the limitations of the current order in the rectifier and the inverter, respectively, may cause the current controller of the inverter to be supplied with a current reference value which is greater than that which is supplied to the current controller of the rectifier. If the difference between the current reference values exceeds the current margin, the current controller of the inverter then tends to increase the current, which can only take place by a further reduction of the direct voltage of the inverter. Thus, in such cases, the control equipment of the inverter strives, with respect to the direct voltage of the inverter, in a direction opposite to the direction aimed at with its control, and it is clear that the risk of this situation arising increases with decreasing current margin.
The phenomena which, as described above, have been able to occur in connection with voltage reductions in the alternating-voltage network of the rectifier may analogously occur in connection with voltage increases in the alternating-voltage network of the inverter. Voltage increases in the alternating-voltage network of the inverter may, especially if the network is weak, still arise in situations in which, because of a fault in the dc transmission, the transferred active power is reduced.